This invention relates to the photochemical deposition of films, and, more particularly, to such deposition of high purity gold films having substantially no organic contamination.
Thin gold films are widely used throughout industry to protect substrates, to give them a distinctive appearance, to form electrical contacts, and for other reasons. A number of different deposition techniques are employed. Gold may be plated electrochemically, spun into substrates from gold-containing organometallic sources, and deposited by various dry deposition techniques wherein no water or other carrier liquid contacts the substrate. In physical vapor evaporation, for example, gold from a metallic source is heated to evaporate gold atoms from the source. The evaporated atoms deposit upon the target substrate surface. Evaporation methods are used to coat areas that are in a line of sight with the source.
Another method is thermally assisted deposition of gold from a gas containing a source of gold. As usually practiced, a gold-containing organometallic compound is mixed with a carrier gas and passed over a heated substrate, so that the organometallic compound decomposes at the surface of the substrate to deposit gold onto the surface. In different variations of thermally assisted deposition, the substrate may be heated by conduction or by irradiation, as with a laser, or from any other suitable source.
Yet another method for depositing gold is by photochemical deposition at ambient or other low temperature from a gold-containing compound. Such low temperature deposition is important industrially, because heating of the substrate is not possible in many applications where there is a need to deposit a gold layer. For example, in some electronics applications the heating of the substrate would cause degradation of the structure of a microelectronic device upon which the gold is to be deposited. Elevated temperatures, whether produced by conduction heating or laser, are therefore unacceptable, and deposition must occur at or near room temperature.
In photochemical deposition, energy for the decomposition is provided from photons interacting with the gold source compound. The gold-containing compound preferentially absorbs light of particular wavelengths, and light within the absorption band is directed at or near the surface of the substrate so that the gold-containing organometallic molecule decomposes to deposit metallic gold on the substrate.
Although a number of different approaches have been attempted to deposit gold photochemically without raising the temperature of the substrate substantially, all suffer from the common problem that the deposited gold layer incorporates a high level of carbon, oxygen, or other impurity. The impurity level is at least several percent, and more typically is on the order of 30 percent or more. This high impurity level increases the electrical resistance of the gold film to unacceptably high levels for many applications. Impure gold is also more porous than pure gold, and forms a less protective layer than does pure gold. Where the gold is present primarily to protect the surface from, for example, oxidation, the availability of a higher purity gold film becomes highly significant. There has been much study of the mechanisms of contamination of the gold film, and the consensus is that fragments of the organometallic compound, produced upon dissociation, are deposited upon the surface and incorporated into the growing gold film along with the gold.
Accordingly, there exists a need for a process for photochemically depositing gold at ambient temperatures, without contamination of the gold deposit by carbon, oxygen, or other contaminants. The present invention fulfills this need, and further provides related advantages.